For a transformer that has a primary coil, a secondary coil, and a core that composes a magnetic circuit of a magnetic field generated by energizing the above-mentioned primary coil, transformers disclosed in JP-A-2010-98207, JP-A-2010-93153, and JP-A-2008-113532 are proposed, for instance.
These transformers are built into a power-supply unit such as a DC-DC converter, and step-up or step-down an input voltage.
When the transformer step-up or step down ratio is adjusted, a ratio of winding number between the primary coil and the secondary coil is adjusted.
Here, although a ratio of transformation of the transformer can be adjusted by changing the number of windings of the primary coil, semiconductor elements etc. that are connected with the primary coil should be changed because an inductance of the primary coil changes, therefore other parts in the power-supply unit that the transformer is assembled require design changing in specifications.
Then, it is desired to avoid changing the transformer design by adjusting the ratio of transformation by changing the number of windings of the secondary coil without changing the number of windings of the primary coil.
However, in the transformers disclosed in JP-A-2010-98207, JP-A-2010-93153, and JP-A-2008-113532, the number of winding of secondary coils for every transformer is one turn, and a transformer with a winding of plural layers is not disclosed.
Further, it is necessary to prepare a plurality of conducting plates that composes the secondary coil and connect the conducting plates in series mutually in order to make the secondary coil with plural windings as if to be composed in a plurality of layers in the direction of a winding axis.
At this time, a projected-shape of the transformer seen from the direction of the winding axis changes when connecting portions are disposed in an outer space of the primary coil and the secondary coil, and a projected area becomes large, as well.
Then, a shape of a space for disposing the transformer in the power-supply unit such as the DC-DC converter equipped with the transformer changes, and there is a problem that an area of the space for disposing the transformer becomes large.
Especially, when changing the design of the power-supply unit that changes only the ratio of transformation of the transformer, it is preferred that only the design of the transformer is changed, and other parts in the power-supply unit remain unchanged completely.
However, when the external outline (projected shape) seen from the direction of the winding axis changes by having the above-mentioned connecting portions provided, this obstructs an efficient design change.